Selecting Concrete Anchor Bolts: Remember 4 and 8

You need concrete anchor bolts – seems simple enough until you get to the hardware store. You can’t simply just use any type of anchor bolt. Some things to consider when purchasing anchor bolts are what type you will need, what kind of load will be put on the structure, or what object is being attached to the concrete.

First off, there are three types of anchor bolts for concrete. The first type is mechanical, they attach to the concrete through friction by expanding against the concrete. The second type is powder-actuated, the most popular to construction because of the ease and speed of installation. The final type is the chemical, which probably does not apply to you unless you are building a bridge and have obtained the proper state certifications.

Now you need to figure out what kind of load will be exerted onto the bolt, will it need to take a shear load or tensile, or both? Whenever a force is exerted on an object parallel to the surface of the concrete, it is referred to as the shear load. Whenever an object is mounted perpendicular, the force exerted is called the tensile load. An example of tensile load would be, a wooden porch mounted to a concrete foundation.

Now that you’ve figured out the type of load, you need to throw in the following factors:

Static loads place the least stress on the anchors, they are steady and constant. An example would be a fire extinguisher mounted to the wall or an electrical box attached to the wall.

Impact loads are pretty self-explanatory, they occur when loads change suddenly. The most common example would be something tossed onto a shelf. The most common example would be something tossed onto a shelf.

Dynamic loads also referred to as vibrating loads, are loads that are constantly changing. An example would be mounting your power tools to a concrete surface, or a surface that is already anchored to concrete. The constant vibration of the load will reduce the anchor’s strength rating.

This is important to figure out, cause this is where the four or eight will come into the factor. Since a manufacturer of the concrete anchor bolts you’re thinking of buying has no idea of the quality of your concrete, they usually make a standard recommendation to use an anchor about four times the weight it will carry for static loads and eight times the weight if it will carry a dynamic or impact load.